What is ISO in Photography

Rajib Mukherjee Avatar
Rajib Mukherjee
18 January, 2023 • Updated 10 days ago
ISO in Photography
ISO refers to the camera sensor’s sensitivity to light. ISO, aperture, and shutter speed are the three pillars of photographic exposure, so getting the ISO just right is crucial.

We have discussed shutter speed and aperture in separate discussions. Today, we’ll focus on ISO and cover what it is and how to master it.

See also: Photography Statistics (2023)


film stock marked 200

Back in the days of film photography, film speed was measured in ASA. There was another standard known as DIN. Later the two were merged, and everything was measured by a common standard that came to be known as ISO.

When I say everything, I’m still referring to film. So you would have film stock marked with ISO numbers that signify their speed. An ISO 200 speed film roll is double that of a roll of ISO 100 film. Therefore, the exposure time taken for an ISO 200 roll of film would be half that of an ISO 100 roll of film.


DSLR camera with figurines of animals and a plate on the table

When it comes to photography, ISO refers to the sensitivity of the sensor or film to light. In digital photography, the term has special significance because ISO is a functionality of the camera that can be changed as per the photographer’s needs. This isn’t the same as in film cameras, where you’re stuck with the film speed you’ve loaded into a camera. Any reference to ISO henceforth will be in relation to digital cameras, unless otherwise mentioned.

The Range of Sensitivity of the Sensor

DSLR camera with display in the hands of the photographer

A photographic sensor has a range of ISO values. These are also sometimes referred to as ISO speeds. The lowest ISO value is usually 100, and the highest is a constantly evolving number. There was a time when ISO 6400 was a phenomenal number.

Not anymore – now we have ISO 102400. It is worth mentioning that higher ISO numbers are more of an academic interest than a practical feature because, at higher ISO, noise creeps in and affects the quality of the images. I will talk about noise in detail later on in this discussion.

ISO numbers can be tweaked on a digital camera – you can use a higher ISO number by simply rotating a dial. When you change the ISO number, the sensor’s sensitivity to light varies. You can tweak the ISO number in one-third stop, half stops, or full stops. This, of course, depends on the camera you’re using and whether that camera offers those options.

All interchangeable lens cameras, including some compact cameras, allow you to change the ISO number. Nowadays, even smartphone cameras also have the option to change the ISO number.

Impact of ISO on the Other Factors of Exposure

DSLR camera photo options

ISO has an exciting relationship with the other exposure factors, namely shutter speed and aperture. For instance, when ISO increases, the shutter speed increases if the aperture remains the same. In other words, the camera requires much less time to expose when the ISO is pushed properly. This has been explained in the examples later in this discussion.

On the other hand, when ISO increases, the aperture decreases if the shutter speed remains the same. This is because when the aperture increases, it’s no longer necessary to keep the same aperture. You can stop the aperture down to get the same exposure.

ISO Sensitivity and Different Shooting Situations

moon in the night sky

Let’s quickly look at how changing the ISO number can help in different shooting situations.

Let’s say you’re shooting a scene at an aperture of f/11 and a shutter speed of 1/50 sec. You don’t want to change the aperture because you want a specific depth of field, and using a larger aperture will create a shallow depth of field (depending on the scene, assuming that there are elements in the middle of the frame and the background that you wish to be reasonably sharp).

However, the lack of light in the scene creates an issue and you cannot get your shutter speed any faster. In a situation like this, increasing the ISO by even one stop will speed up the shutter speed to 1/100 sec, which is again one stop faster than 1/50. Therefore, a higher ISO value is the way to go.

Let’s take another example.

Let’s say that you’re shooting in the evening. You want a portrait image of your significant other. However, the street lights and the neon signs aren’t providing enough light and your shutter speed is stuck at around 1/100.

If you’re shooting with an 85mm lens, you can still click and get a good image without blur. But let’s say that you are using a non-stabilized lens and the maximum aperture is f/4. In a situation like this, when you can’t open the aperture up beyond f/4 and don’t have the luxury of image stabilization to bail you out, pushing the shutter speed and using a faster option is the only solution, which is only made possible by using a higher ISO value.

You can use a higher ISO during daylight as well, as ISO isn’t just a mechanism to boost exposure during the nighttime or when there isn’t enough light to go around. Even with enough light, you can increase the ISO slightly to use a faster shutter speed.

Let’s say you’re trying to photograph a hawk in the daytime while it’s slightly overcast. You wish to capture that moment when the hawk catches a fish and gets out of the water. The shutter speed for such a shot would be less than 1/2000th or even 1/4000th of a second. That will give you enough fast frames to get at least one or two good images. Pushing the ISO by even just one stop should allow you to get the correct shutter speed to capture your action shot.

When to Use Low ISO

city ​​street in daylight

At this stage, you may have a fair idea of when to use low ISO and high ISO. Low ISO is recommended when there is sufficient lighting, such as when you’re shooting outdoors on a well-lit day. Low ISO can simply be the base ISO of your camera, which is the lowest ISO possible on your camera.

Low ISO is also recommended when you can use a slow shutter speed and compensate for the lack of light in the scene by using a long exposure, such as when shooting a cityscape, seascape, or fireworks display. In a situation such as this, you should use a tripod to stabilize your camera and ensure that it doesn’t move when the exposure is being captured.

When to Use A Fast ISO

night sky with stars

Fast ISO is recommended when you need to capture a lot of light in a short period. Such instances can be when you’re shooting indoors and in less-than-ideal lighting. You have to bump up the ISO, otherwise, a slow shutter speed will induce camera shake in low lighting.

Fast ISO is recommended even when shooting in good light when you want to use a fast shutter speed along with capturing a large depth of field. The combination of aperture and shutter speed may not give you the right exposure; thus, you may need to push the ISO to make the exposure possible.

A fast ISO is also useful when you’re shooting astrophotography. Especially when you’re shooting deep space objects like nebulae and ort clouds. It’s ideal to have a very fast shutter speed with a high ISO to ensure that the image is crisp.

Auto ISO

DSLR camera with butterfly and a flower view on the display

Auto ISO is available on most interchangeable lens cameras to dial a higher ISO number automatically when the shutter speed and aperture cannot be tweaked. Let’s use an example to understand how auto ISO works in real-life situations.

Let’s say that you’re shooting at night time and you’re shooting portraits with ambient light. There are a few street lights and a few neon lights to work with. However, with a 50mm f/1.8 (nifty-fifty), you don’t have the luxury of image stabilization.

So, you would first be looking to reduce the shutter speed. But you cannot reduce shutter speed beyond 1/50 sec, otherwise, you risk blurring your images. The solution is to push the ISO.

This is where auto ISO comes into the picture. Auto ISO will automatically push the ISO level to compensate for the lack of light and ensure that your exposure is good. Automatic ISO takes over one exposure parameter and gives the camera the control to dial the ISO number.

Auto ISO is most evident when you’re shooting in any creative mode. You can set a limit as to what should be the minimum shutter speed when auto ISO triggers. Until the lowest shutter speed is reached, auto ISO will not trigger, but as soon as it’s triggered, auto ISO will take over and push the ISO number to balance the exposure.

How Does Auto ISO React When You’re Using Flash?

DSLR camera with flash in the hands of the photographer

Auto ISO works when you’re using flash. Whether you’re using a built-in or external pop-up flash does not matter. The camera will bypass the slowest shutter speed you may have selected when using flash, and the minimum flash sync speed will be used instead.

Some cameras, however, offer the option to fire slow sync. This is done to capture ambient light in dark conditions, which are otherwise suppressed by the flash if it’s firing in normal exposure. The slow shutter sync flash option will use the minimum shutter speed you’ve opted for when setting up auto ISO. This will stop the camera from choosing too slow a shutter speed.

ISO and Noise

Back in the days of film, high-speed film was often associated with capturing what is known as grain. The higher the film speed, the more grain the final images would have. This was tiny specs of white and black spots across the frame.

In modern digital sensor-based imagery, grain is still present – this is referred to as noise. Or, more appropriately, digital noise. Digital noise is very obvious when shooting in low light with high ISO. Look in the shadows for a mishmash of white and black spots to identify noise.

What Factors Determine Noise?

room in sunlight

The advancements in camera technology have allowed us to minimize noise in photos. A few years ago, noise would have been very noticeable at only ISO 1600, making images unusable.

Nowadays, camera noise reduction technology has improved so much that it’s possible to have fairly noiseless images at ISO 3200, with these images even being fit for publication purposes. This has been made possible with the advent of a back-side illuminated sensor and stacked sensor design.

Another factor in the presence of noise is the size of the pixel. Smaller sensors, especially if they’re stacked with pixels, are more likely to produce noise than sensors that are large and yet not stacked with a nearly identical number of pixels. It’s a fact that full-frame sensors with a lower number of pixels are comparatively less affected by noise than smaller sensors that are stacked with pixels (small high-resolution sensors).

Noise Reduction

Adobe Photoshop Lightroom on the display of phone

Noise reduction is the process of reducing noise in an image using software and hardware technology. Today, we’ll introduce you briefly to noise reduction techniques using software. Since I do most of my editing in Adobe Lightroom, I will use the Lightroom technique to showcase how I manage noise in my images.

There are two different kinds of noise in images. One is luminance noise, and the other is color noise. You could have either or both, depending on the image you’re editing. One way to quickly check is to dial down the defaults applied by Lightroom to see if your image has any one particular type. For that, go to “Detail” on the right of the editing panel, where you’ll find “Noise Reduction”.

Tweaking the “Luminance” and “Color” sliders will tell you whether you can salvage any more detail by applying additional noise reduction. One downside to applying noise reduction is that your image is going to lose sharpness and detail.

Rajib Mukherjee Avatar
Rajib is an avid travel photographer and an overall shutterbug. The first time he ever clicked an image was with an Agfa Click IV back in 1984. A medium format film camera. From that auspicious introduction to photography, he has remained hooked to this art form. He loves to test and review new photography gear. Rajib travels quite a lot, loves driving on Indian roads, playing fetch with his Labrador retriever, and loves photography. And yes, he still proudly owns that Agfa Click IV!
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